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Gravitational Waves

Gravitational waves are fluctuations of the curvature of space-time predicted by the Einstein's general theory of relativity. These phenomenon, still not observed directly, is of paramount importance in confirming our understanding of the theory of gravitation as well as of the Universe dynamics.

The Gran Sasso Center for Astroparticle Physics (CFA) promotes studies and research in the field of gravitational waves detection. The CFA participate, through its joint institutes, to different outstanding projects in the field of direct gravitational waves searches.

Research staff of INFN and of the CFA joint institutes Physics Departments of Rome Tor Vergata and L'Aquila Universities, are involved in the ROG Collaboration that runs two gravitational waves detectors: Nautilus and Explorer, both in continuous data taking. Nautilus is an ultracryogenic detector installed at the INFN Frascati National Laboratories. It was the first detector to reach millikelvin temperature. Explorer is a cryogenic detector installed at CERN, with the status of CERN Recognized Experiment. It has been the first modern gravitational wave antenna to perform long-term continuous operation (since 1990). Explorer and Nautilus are part of the international network of resonant-mass detectors which includes Auriga at the INFN Legnaro Laboratories and Allegro at the Louisiana State University, Baton Rouge. The sensitivity to short gravitational wave bursts is h=2 10-19. This sensitivity should allow the detection of signal from gravitational collapses and from coalescing and merging neutron stars/black holes binaries in our galaxy.

The most important activity in Europe in the field of gravitational waves is the Virgo detector, funded by Italy (INFN) and France (CNRS). Virgo is located within the site of EGO, European Gravitational Observatory, in Cascina, near Pisa. The VIRGO collaboration, partecipated by staff of the CFA-node Physics Department of Rome Tor Vergata University, was set up for realizing a powerful gravitational wave detector consisting in a Michelson laser interferometer made of two orthogonal arms being each 3 kilometers long. Multiple reflections between mirrors located at the extremities of each arm extend the effective optical length of each arm up to 120 kilometers. The frequency range of Virgo extends from 10 to 6,000 Hz. This range as well as the very high sensitivity should allow detection of gravitational radiation produced by supernovae and coalescence of binary systems in the milky way and in outer galaxies, for instance from the Virgo cluster. In order to reach the extreme sensitivity required, the whole interferometer attains optical perfection and is extremely well isolated from the rest of the world in order to be only sensitive to gravitational waves. To achieve it, Italian and French scientists involved in the project, have developed most advanced techniques in the field of high power ultrastable lasers, high reflectivity mirrors, seismic isolation, positioning and alignment control. In the field of optics, Virgo uses a new generation of ultrastable lasers, and the most stable oscillator ever built. A specific optical coating facility has been built to produce extremely high quality mirrors combining the highest reflectivity (over 99,999 %), with a surface control at the nanometer scale. To avoid spurious motions of the optical components due to seismic noise, each component is isolated by a 10m high, very elaborate, system of compound pendulums. Because the presence of a residual gas would slightly perturb the measurements the light beam must propagate under ultra high vacuum. The two tubes, 3km long and 1.2m diameter each are actually the largest ultra high vacuum vessels in Europe and the second largest in the world. The environment of the Virgo interferometer is quieter than that of a spacecraft orbiting the earth. The construction of Virgo was completed in June 2003 and the detector is presently taking data. The detector will run day and night listening to all gravitational signals which may arrive at any time, coming from thousands of galaxies.